ELS
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Here's a modified layout.
You can wire the filament winding to the pilot lite Then to the output tubes, a bit less wire that way but it adds another connection that can fail.
Keep the output transformer plate wires as close to the chassis as possible, route it very straight and don't have excess wire in the chassis. And route the grid wires for the output tubes as far away from the plate wires as possible (don't get too close to the screen wires either)... 6L6GC's are very prone to oscillating so the wiring layout in this area is critical. If you still have problems you can put 1.5k resistors in series with the control grid, Place the resistor right next to the tube socket, even 1/4" excess wire can cause a problem at the control grid. So mount some terminal strips to connect the other end of those resistors.
Don't twist the OPT plate wires, I've found this to be more detrimental, probably because in doing so you lift the wire slightly further away from the chassis which in this case seems to actually increase the RF radiation from them. I try to run them pressed up against the chassis, and right next to each other if possible, I sometimes tape them down with aluminum air duct tape for a tiny bit of extra shielding (and mainly to hold them down).
If you opt for the control grid resistors it's far less picky but adding them slightly removes the "singyness" of the amp, it's just a bit but I prefer to not use them if possible
ALso note how the output tube which has the longer grid wire (since it's further away from the circuit) is connected to specifically the 200V rated cap side of the phase inverter, this is intentional; The impedance there for this type of phase inverter is much lower than the other side, resulting in less chance for it to pick up the plate signal and oscillate.
If you need to swap the phase, swap the OPT plate wires or speaker output wires instead.
For the earth-ground terminal, use a split washer not those many-tooth ones, they are the most resistant to getting loose under vibration.
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